The present invention relates generally to electric power steering apparatus and method for controlling the electric power steering apparatus and, more particularly, to an improved electric power steering apparatus and control method therefor which can effectively reduce steering loads on a human operator to permit an enhanced steering feel by imparting a steering torque assist of an electric motor to the steering system of a motor vehicle such as a passenger car.
The electric power steering apparatus are assistance apparatus to provide an automatic steering torque assist by activating a steering assisting motor in response to manual operation of a steering wheel by a human vehicle operator. In the electric power steering apparatus, the steering assisting motor is controlled by a motor control unit, using a steering torque signal output from a steering torque detector section that detects steering torque applied by the vehicle operator to a steering shaft and a vehicle velocity signal output from a vehicle velocity detector section, so as to reduce steering force to be manually applied by the vehicle operator. The motor control unit sets target values of motor currents to be supplied to the steering assisting motor, on the basis of the steering torque signal and vehicle velocity signal, and thereby generates target motor current signals representative of the thus-set target current values. Then, the motor control unit determines a difference or offset between the target motor current signals and motor current signals fed back from a motor current detector section that detects motor currents caused to actually flow through the steering assisting motor, and it then performs a proportional/integral compensation process (PI control) on the determined offset to thereby generate motor control signals for controlling the motor.
Among the conventional electric power steering apparatus is one employing a brushless steering assisting motor, and such an electric power steering apparatus employing the brushless steering assisting motor (hereinafter also referred to as a xe2x80x9cbrushless motorxe2x80x9d) can afford a stable steering torque assist, because the absence of voltage drops between a brush and a commutator can prevent reduction and variation in motor output power. Further, since the brushless motor presents a smaller moment of inertia than the brush-equipped motor, the electric power steering apparatus with such a brushless motor can afford a good steering feel when the steering wheel is turned during high-speed straight travel of the vehicle.
With the electric power steering apparatus employing the brushless motor, it is necessary to control an electric current to be supplied to the motor in accordance with a current rotational angle of the motor. Thus, it has been conventional for the electric power steering apparatus to include, as a feedback-controlling detector unit, a rotational angle detector section for detecting a current rotational angle (electrical angle) of the motor, in addition to the motor current detector section. Output signals from the motor current detector section and rotational angle detector section are fed back to the motor control unit, which in turn generates motor control signals for driving the brushless motor on the basis of PWM (Pulse Width Modulation) control (i.e., PWM-driving the brushless motor) in accordance with the motor currents and rotational angle. Here, the xe2x80x9crotational anglexe2x80x9d, i.e. xe2x80x9celectrical anglexe2x80x9d, of the steering assisting motor is a rotational angle of a rotor detected via magnets provided on the rotor and a magnetism-detecting element provided near the magnet and on the basis of positions of the magnets. Specifically, at an end of the rotor, there are provided a plurality of (e.g., four) magnets, each presenting a pair of N and S magnetic poles, along the circumference of the rotor in such a manner that eight N and S poles are arranged alternately at equal intervals along the circumference of a predetermined portion of the rotor. As the rotor of the steering assisting motor rotates, magnetism produced by the N and S poles alternately acts on the magnetism-detecting element. In the electric power steering apparatus thus constructed, each full rotation of the rotor through a 360xc2x0 mechanical angle produces an electrical angle of four cycles; that is, each quarter turn of the rotor corresponds to a 360xc2x0 electrical angle. Rotational position of the rotor is determined by the electrical angle, and the phases of the motor currents to be supplied to the brushless motor are controlled with reference to the electrical angle.
In the electric power steering apparatus employing the brushless motor, the feedback-based motor control processing carried out by the motor control unit comprises for major steps of: (1) detecting an electrical angle of the rotor; (2) detecting motor currents (e.g., three-phase motor currents) and performing three-phase-to-two-phase conversion (three-phase-to-dq conversion) of the detected motor currents; (3) performing PI control; and (4) performing two-phase-to-three-phase conversion (dq-to-three-phase conversion). These four steps can be further classified into a first group of steps pertaining to feedback control of the motor currents (step (2) and step (3)), and a second group of steps pertaining to control for generating motor control signals using the detected electrical angle (step (1) and step (4)). The motor control unit is implemented by a microcomputer, and thus the four major steps of the feedback-based motor control processing is executed on the basis of computer software.
If the microcomputer (CPU) of the motor control unit is a high-performance microcomputer capable of operating at high speed, the above-mentioned four major steps can always be executed appropriately with no inconveniences even when the steering assisting motor rotates at high speed. However, if the microcomputer (CPU) is of relatively low performance and only capable of operating at low speed, then there would arise the problem that, when the steering assisting motor rotates at high speed, the three-phase sinusoidal waveforms of the motor control signals would deform considerably so that the motor control performance is undesirably lowered.
In view of the foregoing prior art problems, it is an object of the present invention to provide an improved electric power steering apparatus equipped with a brushless steering assisting motor and control method therefor which can reliably avoid degradation in motor control performance even where a microcomputer of low operating speed is employed in a motor control unit.
In order to accomplish the above-mentioned object, the present invention provides an electric power steering apparatus which includes, as a steering assisting motor, a brushless motor capable of being PWM-driven with motor currents of at least three phases to give a steering torque assist to a steering system of a motor vehicle. The electric power steering apparatus of the present invention further comprises: a steering torque detector section for detecting steering torque applied through operation of a steering wheel and generating a steering torque signal indicative of the detected steering torque; a target current setting section for setting target currents on the basis of at least the steering torque signal generated by the steering torque detector section; a motor current detection section for detecting the motor currents to be supplied to the brushless motor and generating motor current signals indicative of the detected motor currents; an electrical angle detector section (rotational angle detection section) for detecting an electrical angle of the brushless motor and generating an electrical angle signal indicative of the detected electrical angle; a motor control unit. The motor control unit comprises: an offset calculation section for calculating offsets between the target currents and the motor currents on the basis of the motor current signals and generating offset signals indicative of the calculated offsets; a first control section for reading the motor current signals generated by the motor current detector section, performing three-phase-to-two-phase conversion on the motor current signals and performing PI control on the offset signals (i.e., PI control based on feedback of the motor current signals); and a second control section for reading the electrical angle signal generated by the electrical angle detection section, performing two-phase-to-three-phase conversion, using the electrical angle signal, to generate motor control signals and, on the basis of the motor control signals, generating the motor currents for driving the brushless motor (i.e., motor drive control based on the electrical angle). Also, in the present invention, an execution cycle time (i.e., interval of time between the occurrence of corresponding portions of repetitively-executed processing cycles) of the control (i.e., PI control based on feedback of the motor current signals) by the first control section is set to be longer than an execution cycle time of the control (i.e., PI control based on feedback of the motor current signals) by the second control section.
The present invention also provides a control method for use with an electric power steering apparatus which detects steering torque applied through operation of a steering wheel, sets target currents on the basis of the detected steering torque, generates motor control signals by detecting and feeding back motor currents, to be supplied to a brushless motor, for comparison to the target currents and then PWM-drives the brushless motor with motor currents of at least three phases in accordance with the motor control signals, the electric power steering apparatus including a motor current detection section for detecting the motor currents to be supplied to the brushless motor and generating motor current signals indicative of the detected motor currents, an electrical angle detection section for detecting an electrical angle of the brushless motor and generating an electrical angle signal indicative of the detected electrical angle, and an offset calculation section for calculating offsets between the target currents and the motor currents and generating offset signals indicative of the calculated offsets. The control method of the present invention comprises: a first step of reading the electrical angle signal generated by the electrical angle detection section; a second step of reading the motor current signals generated by the motor current detector section and performing three-phase-to-two-phase conversion on the motor current signals; a third step of calculating the offset signals by means of the offset calculation section using the motor current signals read by the second step and then performing PI control on the offset signals; and a fourth step of performing two-phase-to-three-phase conversion using the electrical angle signal, read by the first step, to thereby generate the motor control signals and then generate the motor currents. Also, in the present invention, control processing including the first step to the fourth step is executed repetitively with a relatively long cycle time within a range that would not cause inconveniences, and simplified control processing including only the first step and the fourth step is executed repetitively during a period between the repetitive executions of the control processing.
Namely, according to the present invention, the motor control unit repetitively performs the PI control based on the basis of feedback of the motor current signals (the first control section, or the second and third steps) and the motor drive control (the second control section, or the first and fourth steps) while appropriately skipping the PI control. Such inventive arrangements can effectively lessen processing loads on the current feedback control elements of the motor control unit, implemented by a microcomputer, and prevent undesired waveform deformation of the sinusoidal three-phase motor currents to be supplied to the steering-assisting brushless motor during high-speed motor rotation. By thus substantially extending the cycle time of the current feedback control portions, it is possible to lessen the processing loads on the microcomputer in the entire control of the electric power steering apparatus.